[contrib][recovery] Add recovery_directory program

This program takes a file with concatenated zstd frames and splits the
file up by frame. E.g. if `dir.zst` has 4 frames:

```
> ./recover_directory dir.zst recovery/file
Recovering 4 files...
Recovered recovery/file0
Recovered recovery/file1
Recovered recovery/file2
Recovered recovery/file3
Complete
```

contrib/recovery/Makefile

@@ -0,0 +1,35 @@
+# ################################################################
+# Copyright (c) 2019-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under both the BSD-style license (found in the
+# LICENSE file in the root directory of this source tree) and the GPLv2 (found
+# in the COPYING file in the root directory of this source tree).
+# ################################################################
+
+.PHONY: all
+all: recover_directory
+
+ZSTDLIBDIR ?= ../../lib
+PROGRAMDIR ?= ../../programs
+
+CFLAGS     ?= -O3
+CFLAGS     += -I$(ZSTDLIBDIR) -I$(PROGRAMDIR)
+CFLAGS     += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow                 \
+              -Wstrict-aliasing=1 -Wswitch-enum                               \
+              -Wstrict-prototypes -Wundef                                     \
+              -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings      \
+              -Wredundant-decls -Wmissing-prototypes
+CFLAGS     += $(DEBUGFLAGS) $(MOREFLAGS)
+FLAGS       = $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)
+
+.PHONY: $(ZSTDLIBDIR)/libzstd.a
+$(ZSTDLIBDIR)/libzstd.a:
+	$(MAKE) -C $(ZSTDLIBDIR) libzstd.a
+
+recover_directory: recover_directory.c $(ZSTDLIBDIR)/libzstd.a $(PROGRAMDIR)/util.c
+	$(CC) $(FLAGS) $^ -o $@$(EXT)
+
+.PHONY: clean
+clean:
+	rm -f recover_directory

contrib/recovery/recover_directory.c

@@ -0,0 +1,152 @@
+/*
+ * Copyright (c) 2016-2021, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define ZSTD_STATIC_LINKING_ONLY
+#include "util.h"
+#include "zstd.h"
+
+#define CHECK(cond, ...)                                                       \
+  do {                                                                         \
+    if (!(cond)) {                                                             \
+      fprintf(stderr, "%s:%d CHECK(%s) failed: ", __FILE__, __LINE__, #cond);  \
+      fprintf(stderr, "" __VA_ARGS__);                                         \
+      fprintf(stderr, "\n");                                                   \
+      exit(1);                                                                 \
+    }                                                                          \
+  } while (0)
+
+static void usage(char const *program) {
+  fprintf(stderr, "USAGE: %s FILE.zst PREFIX\n", program);
+  fprintf(stderr, "FILE.zst: A zstd compressed file with multiple frames\n");
+  fprintf(stderr, "PREFIX:   The output prefix. Uncompressed files will be "
+                  "created named ${PREFIX}0 ${PREFIX}1...\n\n");
+  fprintf(stderr, "This program takes concatenated zstd frames and "
+                  "decompresses them into individual files.\n");
+  fprintf(stderr, "E.g. files created with a command like: zstd -r directory "
+                  "-o file.zst\n");
+}
+
+typedef struct {
+  char *data;
+  size_t size;
+  size_t frames;
+  size_t maxFrameSize;
+} ZstdFrames;
+
+static ZstdFrames readFile(char const *fileName) {
+  U64 const fileSize = UTIL_getFileSize(fileName);
+  CHECK(fileSize != UTIL_FILESIZE_UNKNOWN, "Unknown file size!");
+
+  char *const data = (char *)malloc(fileSize);
+  CHECK(data != NULL, "Allocation failed");
+
+  FILE *file = fopen(fileName, "rb");
+  CHECK(file != NULL, "fopen failed");
+
+  size_t const readSize = fread(data, 1, fileSize, file);
+  CHECK(readSize == fileSize, "fread failed");
+
+  fclose(file);
+  ZstdFrames frames;
+  frames.data = (char *)data;
+  frames.size = fileSize;
+  frames.frames = 0;
+
+  size_t index;
+  size_t maxFrameSize = 0;
+  for (index = 0; index < fileSize;) {
+    size_t const frameSize =
+        ZSTD_findFrameCompressedSize(data + index, fileSize - index);
+    CHECK(!ZSTD_isError(frameSize), "Bad zstd frame: %s",
+          ZSTD_getErrorName(frameSize));
+    if (frameSize > maxFrameSize)
+      maxFrameSize = frameSize;
+    frames.frames += 1;
+    index += frameSize;
+  }
+  CHECK(index == fileSize, "Zstd file corrupt!");
+  frames.maxFrameSize = maxFrameSize;
+
+  return frames;
+}
+
+static int computePadding(size_t numFrames) {
+  return snprintf(NULL, 0, "%u", (unsigned)numFrames);
+}
+
+int main(int argc, char **argv) {
+  if (argc != 3) {
+    usage(argv[0]);
+    exit(1);
+  }
+  char const *const zstdFile = argv[1];
+  char const *const prefix = argv[2];
+
+  ZstdFrames frames = readFile(zstdFile);
+
+  if (frames.frames <= 1) {
+    fprintf(
+        stderr,
+        "%s only has %u zstd frame. Simply use `zstd -d` to decompress it.\n",
+        zstdFile, (unsigned)frames.frames);
+    exit(1);
+  }
+
+  int const padding = computePadding(frames.frames - 1);
+
+  size_t const outFileNameSize = strlen(prefix) + padding + 1;
+  char* outFileName = malloc(outFileNameSize);
+  CHECK(outFileName != NULL, "Allocation failure");
+
+  size_t const bufferSize = 128 * 1024;
+  void *buffer = malloc(bufferSize);
+  CHECK(buffer != NULL, "Allocation failure");
+
+  ZSTD_DCtx* dctx = ZSTD_createDCtx();
+  CHECK(dctx != NULL, "Allocation failure");
+
+  fprintf(stderr, "Recovering %u files...\n", (unsigned)frames.frames);
+
+  size_t index;
+  size_t frame = 0;
+  for (index = 0; index < frames.size; ++frame) {
+    size_t const frameSize =
+        ZSTD_findFrameCompressedSize(frames.data + index, frames.size - index);
+
+    int const ret = snprintf(outFileName, outFileNameSize, "%s%0*u", prefix, padding, (unsigned)frame);
+    CHECK(ret >= 0 && (size_t)ret <= outFileNameSize, "snprintf failed!");
+
+    FILE* outFile = fopen(outFileName, "wb");
+    CHECK(outFile != NULL, "fopen failed");
+
+    ZSTD_DCtx_reset(dctx, ZSTD_reset_session_only);
+    ZSTD_inBuffer in = {frames.data + index, frameSize, 0};
+    while (in.pos < in.size) {
+        ZSTD_outBuffer out = {buffer, bufferSize, 0};
+        CHECK(!ZSTD_isError(ZSTD_decompressStream(dctx, &out, &in)), "decompression failed");
+        size_t const writeSize = fwrite(out.dst, 1, out.pos, outFile);
+        CHECK(writeSize == out.pos, "fwrite failed");
+    }
+    fclose(outFile);
+    fprintf(stderr, "Recovered %s\n", outFileName);
+    index += frameSize;
+  }
+  fprintf(stderr, "Complete\n");
+
+  free(outFileName);
+  ZSTD_freeDCtx(dctx);
+  free(buffer);
+  free(frames.data);
+  return 0;
+}